Direction finding (DF) antennas utilize spatial separation of antenna elements and measure a phase difference to calculate an angle of arrival to determine the source direction of a radiowave signal or the location of an object. Department of Defense systems may use direction finding for electronic warfare, navigation, search and track pursuits.
Ultra high frequency (UHF) and very high frequency (VHF) direction finding antennas are based on a pseudo Doppler technique requiring a complicated antenna that electronically rotates a view of the antenna. Interferometric direction finding antennas are omni-directional, and use a dipole structure. However, dipole structures result in a high profile (e.g., a half of wavelength) and a limited bandwidth. In some cases, the DF antenna may include four UHF radiating elements and four VHF radiating elements or a total of eight dipole radiating elements. The UHF and VHF radiating elements are each sized and spaced to provide spatial separation suitable for the target wavelengths and lead to large profiles.
Log periodic (LP) antennas provide ultra-wideband (UWB) operation by providing multiple radiating elements and allowing an active radiating region that moves with frequency. The type of radiating elements, the number of radiating elements, and a log scale factor (tau) determine the bandwidth and gain of the LP antenna system. LP antennas provide a cardioid “cos(θ)” element pattern that has directional tendencies. The radiation patterns tend to become more directional as frequency increases.
In antenna design, a phase center is the effective origin of a signal where the radiated field of the signal spreads outward and has equal phase. In direction finding, the distance between phase center of each antenna and the feed center of four antenna elements while maintaining omni-directionality plays a key role in determining ambiguity of the signal.